Protein or peptide drugs are usually administered by injection because of their sluggish absorption via oral administration. Once injected, their in vivo activities last only a short period of time and, for this reason, repeated injections must be administered when a long-term treatment is required. For example, treatment of children suffering from pituitary growth hormone deficiency is carried out by daily injections of recombinant human growth hormone over a period of more than 6 months. Accordingly, a sustained-release formulation which does not require cumbersome daily administrations is highly desirable in such applications.
A typical sustained-release formulation of a protein or peptide drug, e.g., human growth hormone, is prepared by encapsulating the drug in microparticles of a biodegradable polymer matrix material, which slowly releases the drug as the matrix material undergoes in vivo degradation. In this line, extensive studies have been conducted to develop biodegradable polymers suitable for use in sustained-release drug formulations, and biodegradable polyesters such as polylactide, polyglycolide, poly(lactide-co-glycolide), poly-ortho-ester and polyanhydride have been found to be effective in such use [M. Chasin and R. Langer, et al., Biodegradable Polymers as Drug Delivery System, Mercel Dekker (1990) and J. Heller, Adv. Drug Del. Rev., 10, 163 (1993)].
Other studies have also been carried out to develop a sustained-release drug formulation using natural polymer materials such as gelatin, collagen, chitosan, carboxymethyl cellulose, alginate and hyaluronic acid. A natural polymer generally forms a gel when placed in an aqueous environment and this type of highly viscous gel matrix, through which the drug diffuses very slowly, has been used in formulating sustained-release drug compositions.
For example, U.S. Pat. No. 5,416,071 discloses a sustained-release injection formulation of erythropoietin employing a gel containing 0.01% to 3% hyaluronic acid; Japanese Patent Publication No. 1-287041(1989) describes a sustained-released injection formulation of insulin employing a gel formed with 1% hyaluronic acid; and Japanese Patent Publication No. 2-00213(1990) reports a sustained-release formulation of calcitonin, elcanonine or human growth hormone employing a gel containing 5% hyaluronic acid. Similarly, Meyer et al., have developed a sustained-release formulation of granulocyte colony stimulating factor employing a gel containing 0.5 to 4% hyaluronic acid [James Meyer, et al., J. Controlled Release, 35, 67 (1995)].
However, administration of such formulations by injection requires the use of a large bore syringe-needle because a gel containing a few % hyaluronic acid has a high-viscosity in the order of 107 centipoise. Moreover, as the injected gel gets diluted by body fluid, its drug retaining ability is rapidly diminished, and as a result, the sustaining of the drug release lasts no more than 1 day. For instance, Japanese Patent Publication No. 1-287041(1989) discloses that when a sustained-release injection formulation insulin containing 1% hyaluronic acid was administered to rabbits, the therapeutic effect of suppressing the blood glucose level did not last more than 24 hours. Also, the drug concentration in blood was reported to decrease to less than 1/10 of the initial concentration in less than 24 hours when test animals were injected with a formulation of 2% hyaluronic acid containing granulocyte colony stimulating factor [James Meyer, et al., J. Controlled Release, 35, 67(1995)] or a formulation of 1.5% hyaluronic acid containing interferon-α and plasma protein (U.S. Pat. No. 5,416,017). Accordingly, a sustained release drug formulation based on hyaluronic acid gels has the serious drawback that the drug release cannot be maintained for more than 24 hours.
Natural hyaluronic acid or an inorganic salt thereof dissolves only in water. Hyaluronic acid-benzyl ester HYAFF™, on the other hand, dissolves not in water but in an organic solvent, e.g., dimethylsulfoxide. Drug compositions, comprising solid microparticles of such hydrophobic hyaluronic acid derivatives and drugs encased therein have been prepared by the conventional emulsion-solvent extraction method [N. S. Nightlinger, et al., 9Proceed. Intern. Symp. Control. Rel. Bioact. Mater., 22nd, Paper No. 3205 (1995); L. Ilum, et al., J. Controlled Rel., 29, 133 (1994)]. Such preparation is typically carried out as follows: A protein drug is dispersed in a dimethylsulfoxide solution of hyaluronic acid-benzyl ester and the dispersion thus obtained is added to a mineral oil to form an emulsion. An organic solvent, e.g., ethylacetate, is added to the emulsion to extract dimethylsulfoxide; and microparticles consisting of the drug and hyaluronic acid-benzyl ester are recovered therefrom.
However, this method has the problem that the protein drug may be denatured through its contact with the organic solvent or with hydrophobic hyaluronic acid-benzyl ester. In fact, a microparticular composition of granulocyte macrophage-colony stimulating factor (GM-CSF) prepared by using a fully esterified hyaluronic acid derivative was reported to release only about 25% of GM-CSF during the first few days and none after 17 days [N. S. Nightlinger, et al., proceed. Intern. Symp. Control. Rel. Bioact. Mater., 22nd, Paper No. 3205(1995)]. In this case, a major portion of the protein drug was lost, most likely due to denaturation thereof through its interaction with hyaluronic acid-benzyl ester and/or the organic solvent.